Such servos conventionally include a casing having an axis of symmetry, divided in leaktight fashion by a movable wall structure into a front chamber permanently connected to a source of partial vacuum, and a rear chamber connected selectively to the front chamber or to the outside atmosphere by a three-way valve actuated by an axial control rod subjected to the action of a return spring located in an annular space between the control rod and a rear tubular part of the movable wall structure, projecting out of the casing, the three-way valve being located in this rear tubular part.
These servos are usually located in the engine compartment of the vehicle, on its bulkhead, so that the rod for controlling the servo projects into the passenger compartment so that it can be actuated by the driver. It follows that the rear central part of the movable wall, including the three-way valve, also projects into the passenger compartment.
As a result, when the driver actuates the brake pedal connected to the control rod of the servo, the latter sucks air at atmospheric pressure in from the passenger compartment of the vehicle. The air set in motion by the operation of the servo therefore generates noises which are wholly transmitted into the passenger compartment of the vehicle.
Attempts have been made to provide various solutions to this problem. For example, document FR-A-2,551,009 provides means for guiding the air in the immediate surroundings of the three-way valve, upstream and downstream of the latter.
Document FR-A-2,516,880 provides a silencer device made from crosslinked foam, located inside the rear central part of the movable wall structure, this device including an outer surface applied against the inner surface of this rear central part, and the inside of which is formed with uninterrupted passage openings pointing in the direction of the control rod to give a passage for the flow of air through the device.
Document DE-A-3,924,672 for its part provides a servo in which the intake for air at atmospheric pressure is situated in the engine compartment of the vehicle.